Tunnel junctions with ferromagnetic electrodes are known to exhibit a large magnetoresistance. This makes magnetic tunnel junctions a strong candidate for applications requiring magnetic sensors, such as high areal density magnetic recording heads, magnetic random access memory (MRAM), and field sensors. However, these applications require junctions with low resistance.
To date, oxidized aluminum is the most widely used material from which the thin insulating tunnel barrier necessary for making low-resistance junctions can be formed. In this case, the aluminum thin film is deposited on the hard layer of the tunnel junction and then oxidized to form the insulating barrier. While many groups have successfully fabricated low-resistance junctions using this method, there are several challenges involved when scaling this technique into a production-level process.
First, tunnel junction performance is strongly dependent upon aluminum thickness when the insulating barriers are very thin. Thus, thin film deposition tools must be able to deposit xcx9c7 xc3x85 of aluminum with sub-angstrom accuracy and precision over an entire wafer. Second, there is evidence that aluminum interdiffuses into conventional electrode materials, such as CoFe and NiFe, which makes the fabrication of monolayer thick barriers difficult. Third, oxidation of the bottom junction electrode must be minimized. Lastly, the oxidized aluminum barriers intermix with CoFe and NiFe, causing irreversible damage, when exposed to temperatures above 250-300xc2x0 C. This can be a problem because wafer processing can expose on-wafer devices to temperatures of 250xc2x0 C. and above.
Magnetic tunnel junction performance is strongly dependent upon the thickness and quality of the insulating barrier and its interaction with the ferromagnetic electrodes. Current state-of-the-art junctions have resistance-area products (a measure of the intrinsic barrier resistance) of R*A=xcx9c10 xcexa9-xcexcm2, which is consistent with an alumina barrier between one and two monolayers thick. However, even this low of a resistance is potentially too high for magnetic recording heads. Attempts to push the oxidized aluminum tunnel barrier technology to monolayer thickness have yielded junctions with pinholes or gaps across the barrier. The end result is that the junctions exhibit near zero magnetoresistance and extremely poor magnetic properties.
It is known in the art that small ferromagnetic nanocrystals can be formed out of materials such as Co and FePt. The size of these nanocrystals can, in principle, be tuned by varying preparation conditions, but are typically on the order of xcx9c10 nm in diameter. Furthermore, they have a nearly monodisperse distribution in size (xcex4xcx9c5%). The nanocrystals can be applied to wafers by dissolving the particles in a solvent, spreading the solvent/nanocrystal solution on a wafer surface, and inducing a controlled evaporation of the solvent. Using this technique, nanocrystal superlattices one to three monolayers thick have been achieved.
It is desirable to develop a more robust process for fabricating low resistance ferromagnetic tunnel junctions for use in magnetic sensor application to eliminate or reduce the above described difficulties. Further, it would be desirable to replace the oxidized aluminum monolayer with a monolayer composed of ferromagnetic nanoparticles.
Disclosed is a magnetic sensor utilizing a tunnel junction which is an improvement over prior art tunnel junctions. The improved tunnel junction ideally consists of a monolayer of ferromagnetic nanoparticles, such as FePt. Surrounding the nanoparticles is an insulating layer composed of oleic acid or other carbonaceous coating. Non-magnetic electrical leads deposited to the top and bottom of the tunnel junction are used to pass current through the device in a perpendicular-to-the-plane (CPP) configuration.